What are Cold Frames?

Cold frames are simple structures used in gardening as a way to extend the growing season. They are made up of many different types of materials, from wood to cement blocks, and topped with a frame of glass, plastic sheeting or Plexiglas. Their function is to provide a warm, insulated, protected place to prepare seedlings sowed indoors for the early spring or fall conditions outside.

This process of preparing fragile seedlings for the outdoors is called hardening off by gardeners. Using cold frames can allow a gardener to plant seedlings outside up to four to six weeks before the last frost. Without cold frames, gardeners must wait to plant seeds outdoors until after the last frost to avoid losing them to freezing weather. Cold frames give plants a warm, humidified, protected environment to thrive in.

As with nearly any gardening tool, cold frames may be purchased online or at gardening supply stores. For the do-it-yourselfer, a simple wood frame may be constructed, and then topped with an old window or frame with plastic stretched over it. Another way to build a simple cold frame is to stack concrete blocks, then top them with a window or plastic covered frame. Pre-fab basement window well covers can also be used to cover simple cold frames. Bales of hay may be arranged to form a cold frame, then covered with a window or wood frame.

When using a cold frame with one side that is lower than the other, make sure that the lower side is facing south in order to maximize sun exposure. All cold frames should be oriented east to west to maximize the sunlight. Cold frames often become too hot, especially during a warm spell or sunny days. They shouldn’t be allowed to rise above 80° Fahrenheit (27° Celsius) in order to avoid excessive moisture and heat.

Do-it-yourself cold frames can be vented by propping the top up. More expensive and complicated cold frames may have automated venting windows so that they vent on their own when they become too hot. Venting cold frames is important to reduce the humidity inside. High levels of moisture and inadequate air circulation can lead to foliar diseases.

Cold frames must be small enough for the gardener to reach across for weeding and other plant care. The foundation of the cold frames should be sunk below the soil level for extra insulation. Cold frames can also be set up against a building or wall to provide protection from the elements.

Gardeners with plenty of room for designated cold frames may opt for more permanent structures, while those with limited gardening space may choose lightweight, portable cold frames that can be moved from bed to bed. Cold frames can also serve as mini greenhouses, or they can be used open for additional bedding space.